Process for producing phosphinates

ABSTRACT

The present invention relates primarily to a process for producing particular phosphinates (phosphonous acid monoesters) and use thereof for producing biologically active substances which may be used in the pharmaceutical or agrochemical sector, preferably for producing phosphorus-containing amino acids.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/763,398, filed Mar. 26, 2018, which claims the benefit ofInternational Application No. PCT/EP2016/072786, filed Sep. 26, 2016,which claims priority to European Patent Application No. 15187469.0,filed Sep. 29, 2015, the content of these applications is hereinincorporated by reference in their entirety.

BACKGROUND

The present invention relates primarily to a process for producingphosphinates (phosphonous acid monoesters) of the defined formula (I)below and use thereof for producing biologically active substances whichmay be used in the pharmaceutical or agrochemical sector, preferably forproducing phosphorus-containing amino acids.

DESCRIPTION OF RELATED ART

Phosphinates (phosphonous acid monoesters), alkyl phosphinates forexample (alkylphosphonous acid monoesters), are valuable intermediatesin various industrial fields, in particular for producing biologicallyactive substances which can be employed in the pharmaceutical oragrochemical sector.

U.S. Pat. No. 4,168,963, for example, describes a wide variety ofphosphorus-containing herbicidally active compounds, among which inparticular phosphinothricin(2-amino-4-[hydroxy(methyl)phosphinoyl]butanoic acid; common name:glufosinate, referred to hereinbelow as glufosinate) and the saltsthereof have attained commercial importance in the agrochemical sector.

Methods for producing intermediates for the synthesis of suchphosphorus-containing herbicidally active compounds, in particular ofglufosinate, are described in U.S. Pat. Nos. 4,521,348, 4,599,207 and6,359,162B1 for example.

The production of monoalkyl phosphinates is known to those skilled inthe art and may be effected according to processes known from theliterature, for example in accordance with U.S. Pat. Nos. 3,914,345;4,474,711 or 5,128,495).

The production of monoalkyl phosphinates starting from particularhalophosphorus compounds is described, for example, in U.S. Pat. No.4,485,052, GB 1461 376 and GB 1 490 835.

The production of dialkyl alkylphosphonites is described, for example,in U.S. Pat. No. 5,166,385.

Furthermore, saponification reactions of certain dialkylalkylphosphonites to give alkyl phosphinic acids or monoesters thereofare known, as described for example in Applied Spectroscopy 1968, 22,95-98, J. Med. Chem. 1988, 31, 204-212, J. of General Chem. of the USSR1962, 32, 3288-3296, Synthetic Communications 2003, 33, 1665-1674 and J.Organomet. Chem. 1997, 529, 135-142.

J. Am. Chem. Soc. 1958, 80, 5937-5940 describes the transesterificationof certain dialkyl alkylphosphonites.

The processes from the prior art for producing alkyl phosphinates(alkylphosphonous acid monoesters) still have disadvantages however,such as insufficient purities and/or yields of alkyl phosphinates(alkylphosphonous acid monoesters), an excessive fraction of co-productsor by-products, excessively complicated purification or isolation of thealkyl phosphinates (alkylphosphonous acid monoesters) and/or reactionconditions that are excessively difficult in terms of process or planttechnology.

Alkylphosphonous acid mono-C₁-C₃-esters are preferably not used on anindustrial scale due to their low stability and their hazard potential.In the large scale synthesis of glufosinate ammonium, therefore, butylmethylphosphinate is an important intermediate.

SUMMARY

The object of the present invention, therefore, was to find a processfor producing phosphinates (phosphonous acid monoesters), particularlyalkyl phosphinates (alkylphosphonous acid monoesters), which affords thephosphinates (phosphonous acid monoesters) in improved yields and/orgives rise to a lower fraction of co-products or by-products, and inaddition preferably enables an improved reaction regime, for example, inrelation to aspects relevant to safety, environment and/or quality, andare thus preferably also suitable for performance on an industrialscale.

The process according to the invention described below achieves thisobject.

The present invention provides a process for producing phosphinates(phosphonous acid monoesters) of formula (I)

characterized in that a compound of formula (II)

is reacted with a compound of formula (III)R²—OH  (III)wherein in each case:R¹ represents (C₁-C₁₂)-alkyl, (C₁-C₁₂)-haloalkyl, (C₆-C₁₀)-aryl,(C₆-C₁₀)-haloaryl, (C₇-C₁₀)-aralkyl, (C₇-C₁₀)-haloaralkyl,(C₄-C₁₀)-cycloalkyl or (C₄-C₁₀)-halocycloalkyl,R² represents (C₃-C₁₂)-alkyl, (C₃-C₁₂)-haloalkyl, (C₆-C₁₀)-aryl,(C₆-C₁₀)-haloaryl, (C₇-C₁₀)-aralkyl, (C₇-C₁₀)-haloaralkyl,(C₄-C₁₀)-cycloalkyl or (C₄-C₁₀)-halocycloalkyl,R³ and R⁴ each independently of one another represent methyl or ethyl,in the presence of an acidic catalyst and in the presence of water.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The desired phosphinates (phosphonous acid monoesters) of formula (I)are obtained in excellent yield and in very high purity with the processaccording to the invention by the reaction of the compound of formula(II) with an alcohol of formula (III), at the same time in the presenceof a minimum amount of water and in the presence of an acidic catalyst.

In the process according to the invention, partial hydrolysis andtransesterification of the compounds of formula (II) takes place insitu, preferably in a one-pot reaction. In the process according to theinvention, particularly in one of the configurations of the processaccording to the invention designated as preferred or as particularlypreferred, the phosphinates (phosphonous acid monoesters) of formula (I)are obtained in virtually quantitative yield and in very high purity.The process according to the invention is (from a process engineeringperspective) very simple and suitable for performing on a large scale.

Overall, the process according to the invention, and also the processfor producing glufosinate described below, form fewer undesiredsecondary components so that these processes are more efficient and moreenergy-saving.

The respective alkyl radicals of the radicals R¹ and R² may have astraight-chain or branched-chain (branched) carbon skeleton.

The expression “(C₁-C₄)-alkyl”, by way of example, is a brief notationfor an alkyl radical having 1 to 4 carbon atoms, i.e. encompasses theradicals methyl, ethyl, 1-propyl, 2-propyl, 1-butyl, 2-butyl,2-methylpropyl or tert-butyl. General alkyl radicals having a largerspecified range of carbon atoms, for example “(C₁-C₆)-alkyl”,correspondingly also encompass straight-chain or branched alkyl radicalshaving a greater number of carbon atoms, i.e. also the alkyl radicalshaving 5 and 6 carbon atoms.

“Halogen” preferably refers to the group consisting of fluorine,chlorine, bromine and iodine. Haloalkyl, haloaryl, haloaralkyl andhalocycloalkyl respectively refer to alkyl, aryl, aralkyl and cycloalkylpartially or completely substituted by identical or different halogenatoms, preferably from the group fluorine, chlorine and bromine, inparticular from the group fluorine and chlorine. Thus haloalkylencompasses for example monohaloalkyl (=monohalogenalkyl), dihaloalkyl(=dihalogenalkyl), trihaloalkyl (=trihalogenalkyl) or else perhaloalkyl,for example CF₃, CHF₂, CH₂F, CF₃CF₂, CH₂FCHCl, CCl₃, CHCl₂, CH₂CH₂C₁.The same applies for the other halogen-substituted radicals.

The process according to the invention preferably relates to producingphosphinates (phosphonous acid monoesters) of formula (I)

-   where in each case-   R¹ represents (C₁-C₆)-alkyl, (C₁-C₆)-haloalkyl, (C₆-C₈)-aryl,    (C₆-C₈)-haloaryl, (C₇-C₁₀)-aralkyl, (C₇-C₁₀)-haloaralkyl,    (C₅-C₈)-cycloalkyl or (C₅-C₈)-halocycloalkyl,-   and-   R² represents (C₃-C₈)-alkyl, (C₃-C₈)-haloalkyl, (C₆-C₈)-aryl,    (C₆-C₈)-haloaryl, (C₇-C₁₀)-aralkyl, (C₇-C₁₀)-haloaralkyl,    (C₅-C₈)-cycloalkyl or (C₅-C₈)-halocycloalkyl.

The process according to the invention preferably relates to producingphosphinates (phosphonous acid monoesters) of formula (I)

where in each case

R¹ represents (C₁-C₄)-alkyl, (C₁-C₄)-haloalkyl or (C₆-C₈)-aryl,preferably methyl or ethyl,

and

R² represents (C₃-C₆)-alkyl or (C₃-C₆)-haloalkyl, preferably(C₃-C₆)-alkyl, preference among these in turn being given to C₄-alkyl orC₅-alkyl.

The process according to the invention relates in particular toproducing certain alkyl phosphinates (alkylphosphonous acid monoesters)of formula (I), in which particularly preferably in formula (I)

R¹ represents methyl, and

R² represents (C₃-C₆)-alkyl, particular preference in turn being givento (C₄-C₅)-alkyl.

The implementations which follow and the embodiments of the processaccording to the invention characterized as preferable/particularlypreferable apply in particular for the reaction of a compound of formula(I), in which R¹ represents methyl and R² represents (C₃-C₆)-alkyl.

An advantageous process according to the invention is characterized inthat the total amount of water used is at least 0.8 molar equivalents,preferably at least 0.9 molar equivalents, more preferably at least 0.95molar equivalents, based in each case on the total amount of compoundsof formula (II) used.

A preferred process according to the invention is characterized in thatthe total amount of water used is 1 to 5 molar equivalents, based on thetotal amount of compounds of formula (II) used.

A particularly preferred process according to the invention ischaracterized in that the total amount of water used is 1 to 3 molarequivalents, especially preferably 1 to 2 molar equivalents, based ineach case on the total amount of compounds of formula (II) used.

A preferred process according to the invention is characterized in thatthe total amount of compounds of formula (III) used is 1 to 25 molarequivalents, preferably 2 to 20 molar equivalents, based in each case onthe total amount of compounds of formula (II) used.

A particularly preferred process according to the invention ischaracterized in that the total amount of compounds of formula (III)used is 3 to 15 molar equivalents, preferably 4 to 12 molar equivalents,preferably 5 to 10 molar equivalents, based in each case on the totalamount of compounds of formula (II) used.

A preferably recovered excess of the compound (III) may subsequently bereused in the same reaction without further purification.

In a preferred configuration in the process according to the invention,preference is given to using initially a portion of the overall totalamount of alcohols of formula (III) used in the reaction. By way ofpreference, initially a proportion of 20 to 80% by weight, preferably aproportion of 30 to 70% by weight, more preferably a proportion of 40 to60% by weight of the overall total amount of the alcohols of formula(III) used is mixed and reacted with the compounds of formula (II),water and acidic catalyst, before the residual amount of the overalltotal amount of the alcohols of formula (III) used is added to theresulting reaction mixture.

A preferred process according to the invention is characterized in thatthe pKa of the acidic catalyst under standard conditions (273.15 K and100 kPa) is less than 3, preferably less than 2, and preferably lessthan 1.

In one process according to the invention, preference is given to acidiccatalysts selected from H₃PO₃, H₂SO₄, HCl, HBr, HClO₄, dichloroaceticacid, trichloroacetic acid, trifluoroacetic acid, methanesulphonic acid,trifluoromethanesulphonic acid, benzenesulphonic acid andp-toluenesulphonic acid.

In one process according to the invention, preference is given to acidicheterogeneous (solid) catalysts selected from acidic ion exchangers,acidic polysiloxanes and acidic zeolites.

A preferred process according to the invention, therefore, ischaracterized in that one or the acidic catalyst is selected from thegroup consisting of H₃PO₃, H₂SO₄, HCl, HBr, HClO₄, dichloroacetic acid,trichloroacetic acid, trifluoroacetic acid, methanesulphonic acid,trifluoromethanesulphonic acid, benzenesulphonic acid,p-toluenesulphonic acid, acidic ion exchangers, acidic polysiloxanes andacidic zeolites.

A particularly preferred process according to the invention ischaracterized in that the pKa of the acidic catalyst under standardconditions (273.15 K and 100 kPa) is less than 0 (zero), and preferablyless than −1.

Particular preference is given to acidic catalysts selected from H₂SO₄,HCl, methanesulphonic acid, trifluoromethanesulphonic acid andp-toluenesulphonic acid.

Particularly preferred acidic heterogeneous (solid) catalysts are thosehaving sulphonic acid groups, i.e. —SO₃H groups.

The overall amount used (total amount) of the acidic catalyst(s) in theprocess according to the invention is not critical for carrying out theprocess according to the invention, being preferably 0.1 to 10% byweight, preferably 0.5 to 5% by weight, based in each case on theoverall amount used (total amount) of the compounds of formula (II).

A particular advantage and a particularly preferred configuration of theprocess according to the invention are characterized in that thereaction is carried out as a one-pot reaction, i.e. reaction ofcompounds (II) and (III) to give compound (I) is particularly preferablycarried out in accordance with the invention without isolation ofintermediates.

The process according to the invention is preferably carried out suchthat the reaction is carried out at a temperature in the range from 30to 140° C., preferably at a temperature in the range from 40 to 130° C.,more preferably at a temperature in the range from 50 to 120° C. andparticularly preferably at a temperature in the range from 60 to 110° C.

The process according to the invention enables the production of thephosphinates (phosphonous acid monoesters) of formula (I) under mildreaction conditions and in a manner that is simpler to carry out interms of process/plant engineering. The phosphinates (phosphonous acidmonoesters) of formula (I) can therefore be obtained more easily inprocess engineering terms, in better yields and in high purity.

The purity of the desired products of formula (I) after purification,for example, after purification by distillation, is regularly greaterthan 95%.

The process according to the invention is therefore particularlysuitable for carrying out on an industrial scale, i.e. the processaccording to the invention is preferably carried out in a manner inwhich at least an amount of 100 kg of compounds of formula (II) is usedin the process according to the invention, preferably at least an amountof 250 kg of compounds of formula (II), more preferably at least anamount of 500 kg of compounds of formula (II).

The phosphinates (phosphonous acid monoesters) of formula (I) formed maybe used as starting materials for the synthesis of phosphorus-containingamino acids such as glufosinate and glufosinate salts.

Preferred configurations of the process according to the invention forproducing the compounds of formula (I) by reacting a compound of formula(II) with the alcohol of formula (III) are characterized in that

the total amount of water used is 1 to 3 molar equivalents, based on thetotal amount of compounds of formula (II) used,

and

the total amount of compounds of formula (III) used is 2 to 20 molarequivalents, more preferably the total amount of compounds of formula(III) used is 3 to 15 molar equivalents, based in each case on the totalamount of compounds of formula (II) used.

Further preferred configurations of the process according to theinvention for producing the compounds of formula (I) by reacting acompound of formula (II) with the alcohol of formula (III) arecharacterized in that

the total amount of water used is 1 to 2 molar equivalents, based on thetotal amount of compounds of formula (II) used,

and

the total amount of compounds of formula (III) used is 4 to 12 molarequivalents, more preferably the total amount of compounds of formula(III) used is 5 to 10 molar equivalents, based in each case on the totalamount of compounds of formula (II) used.

Particularly preferred configurations of the process according to theinvention for producing the compounds of formula (I) by reacting acompound of formula (II) with the alcohol of formula (III) arecharacterized in that

the total amount of water used is 1 to 3 molar equivalents, based on thetotal amount of compounds of formula (II) used,

the total amount of compounds of formula (III) used is 2 to 20 molarequivalents, more preferably the total amount of compounds of formula(III) used is 3 to 15 molar equivalents, based in each case on the totalamount of compounds of formula (II) used,

the acidic catalyst(s) are selected from the group consisting of H₂SO₄,HCl, methanesulphonic acid, trifluoromethanesulphonic acid,p-toluenesulphonic acid and acidic heterogeneous (solid) catalystshaving sulphonic acid groups, i.e. —SO₃H groups,

the reaction is carried out at a temperature in the range from 30 to140° C., preferably at a temperature in the range from 40 to 130° C.,

and the overall amount used (total amount) of the acidic catalyst(s) ispreferably 0.1 to 10% by weight, preferably 0.5 to 5% by weight, basedin each case on the overall amount used (total amount) of compounds offormula (II).

Particularly preferred configurations of the process according to theinvention for producing the compounds of formula (I) by reacting acompound of formula (II) with the alcohol of formula (III) arecharacterized in that

the total amount of water used is 1 to 2 molar equivalents, based on thetotal amount of compounds of formula (II) used,

the total amount of compounds of formula (III) used is 3 to 15 molarequivalents, more preferably the total amount of compounds of formula(III) used is 4 to 12 molar equivalents, based in each case on the totalamount of compounds of formula (II) used,

the acidic catalyst(s) are selected from the group consisting of H₂SO₄,HCl, methanesulphonic acid, trifluoromethanesulphonic acid,p-toluenesulphonic acid and acidic heterogeneous (solid) catalystshaving sulphonic acid groups, i.e. —SO₃H groups,

the reaction is carried out at a temperature in the range from 50 to120° C., preferably at a temperature in the range from 60 to 110° C.,

and

the overall amount used (total amount) of the acidic catalyst(s) is 0.5to 5% by weight, based on the overall amount used (total amount) ofcompounds of formula (II).

The process according to the invention may be carried out either in adiscontinuous process regime (for example in a semibatch mode ofoperation) or else in a continuous process regime (for example in acontinuously operated stirred tank).

The process according to the invention is preferably carried out underinertization, preferably in a protective gas atmosphere. Preferredprotective gases in this case are nitrogen/argon.

It is further possible to carry out the process according to theinvention under superatmospheric pressure or under reduced pressure.

The process according to the invention may be carried out in an inertdiluent or diluent-free.

If the process according to the invention is carried out in an inertdiluent, i.e. stable under the reaction conditions, the diluent ispreferably selected from the group consisting of hydrocarbons (preferredin this case are alkanes and aromatic hydrocarbons), halogenatedhydrocarbons (preferred in this case are haloalkanes and halogenatedaromatic hydrocarbons), ethers (preferred in this case are cyclicethers, alkyl alkyl ethers and aryl alkyl ethers), carboxylic acidesters (preferred in this case are alkyl alkyl esters and aryl alkylesters) and carboxamides, and mixtures thereof.

Usable optional diluents in the process according to the invention areinert organic solvents such as heptane, toluene, xylene, ethylbenzene,chlorobenzene, dichlorobenzene, anisole, dimethyl formamide (DMF),dimethylacetamide, N-methyl-2-pyrrolidone (NMP) or mixtures of theseorganic solvents.

Preferred diluents in this case are heptane, toluene, xylene,ethylbenzene, cumene, chlorobenzene, dichlorobenzene, anisole andmixtures thereof.

More preferably, the diluent forms an azeotropic mixture with thealcohol(s) methanol and/or ethanol resulting from the residues R³ or R⁴of the compound of formula (II) in the process according to theinvention, which can preferably be removed from the reaction mixture bydistillation, i.e. can be distilled off. Preferred diluents in thisregard are aromatic hydrocarbons, preference in turn being given toalkylbenzenes such as toluene, xylene and/or ethylbenzene.

In a preferred configuration, the process according to the invention iscarried out without addition of a diluent.

Glufosinate salts in the context of the present invention are preferablyammonium salts, phosphonium salts, sulfonium salts, alkali metal saltsand alkaline earth metal salts of glufosinate.

Especially preferred in the context of the present invention areglufosinate, glufosinate sodium and glufosinate ammonium.

Finally, the present invention also relates to the use of a compound offormula (I), produced in accordance with a process according to theinvention, as defined above, for producing biologically activesubstances which may be used in the pharmaceutical or agrochemicalsector, preferably for producing phosphorus-containing amino acids,particularly for producing glufosinate or glufosinate salts, in thiscase particularly glufosinate, glufosinate sodium or glufosinateammonium.

The process for producing glufosinate and/or glufosinate salts may beeffected in similar fashion as described for example in U.S. Pat. No.4,521,348.

The examples which follow elucidate the present invention.

EXAMPLES

All data are based on weight unless otherwise stated.

Example 1: 1-Butyl methylphosphinate (methanephosphonous acidmono-n-butyl ester)

38.45 g (0.2684 mol) of diethyl methylphosphonite (purity 95%) and 1.5 gof Amberlyst® 15 (strongly acidic catalyst, synthetic ion exchangeresin) in 200 ml of toluene were initially charged in a stirred flaskunder an argon atmosphere. 4.95 g (0.275 mol) of water and 50 g (0.675mol) of 1-butanol were added while stirring. Subsequently, the mixturewas stirred at 50° C. for 30 min and then heated to reflux (initiallyca. 83° C.).

Low boiling components (ethanol and toluene) were then distilled off viaa distillation attachment with a short Vigreux column. After 4 hoursanother 50 g (0.675 mol) of 1-butanol were added and further low boilingcomponents were distilled off. Finally, at 140° C. at the bottom/113° C.at the top, a mixture of 37% 1-butanol and 63% toluene (GC analysis) wasremoved.

After cooling and removal of the acidic catalyst, 64.3 g remained of amixture of 55.6% 1-butyl methylphosphinate, 41.3% 1-butanol and 3.1%toluene (GC analysis against standard), corresponding to a yield of35.75 g (0.263 mol)=97.9% of theory.

By means of a fractionated fine distillation under reduced pressure,1-butyl methylphosphinate was obtained at a purity of 98.5%.

Analysis: ³¹P-NMR (CDCl₃)

1-butyl methylphosphinate 33.9 ppm

diethyl methylphosphonite 177.8 ppm

Example 2: 1-Pentyl methylphosphinate (methanephosphonous acidmono-n-pentyl ester)

To 20 g (0.145 mol) of diethyl methylphosphonite (purity 99%) were added2.62 g (0.145 mol) of water and 64.1 g (0.727 mol) of 1-pentanol and themixture was reacted in the presence of 0.1 g of 96% sulphuric acidwithout additional solvent, wherein the mixture was stirred at the startat 50° C. for 30 minutes, and then the internal temperature wasincreased gradually over 3 hours up to the reflux temperature of1-pentanol by the end. At the same time, low boiling components weredistilled off, at the end only 1-pentanol (GC analysis), which wasmostly distilled off.

According to analysis by GC, the resulting reaction mixture no longercontained reactant.

After fine distillation under reduced pressure, 21.8 g (0.139 mol) of1-pentyl methylphosphinate (purity 96%) were obtained from the crudeproduct, corresponding to a yield of 95.8% of theory.

Analysis: ³¹P-NMR (CDCl₃): 1-pentyl methylphosphinate 34.11 ppm

Example 3: 1-Butyl phenylphosphinate (phenylphosphonous acidmono-n-butyl ester)

To 5.0 g (24.7 mmol) of diethyl phenylphosphonite (purity 98%) underargon were added 9.72 g (131.17 mmol) of 1-butanol, 0.445 g (24.7 mmol)of water and 0.3 g of Amberlyst® 15 (strongly acidic catalyst, syntheticion exchange resin) at 20° C. without additional diluent and the mixturewas heated to reflux. Low boiling components were then distilled off viaa distillation attachment with a short Vigreux column up to a bottomtemperature of 117-120° C. A further 8.1 g (109.31 mmol) of 1-butanolwere then added and low boiling components further distilled off at abottom temperature of about 120° C. The overall reaction lasted in totalabout 5.5 hours. The profile was monitored by GC analysis. 7.95 g ofcrude product remained at the end (according to ¹H-NMR 58.3%, theresidue was 1-butanol). This corresponded to a yield of 94.6% of theory.

Pure 1-butyl phenylphosphinate could be obtained via a fine distillationunder reduced pressure.

Analysis: ³¹P-NMR (CDCl₃)

1-butyl phenylphosphinate 25.1 ppm

diethyl phenylphosphonite 151.2 ppm

Example 4: 2-Methylpropyl phenylphosphinate (phenylphosphonous acidmono-2-methylpropyl ester)

Analogously to Example 2 above, diethyl phenylphosponite (purity 98%)was reacted with water (1 molar equivalent) and isobutanol (8 molarequivalents) in the presence of a catalytic amount of concentratedsulphuric acid (2 mol %, based on the amount of diethylphenylphosphonite used).

2-Methylpropyl phenylphosphinate was obtained in a yield of 95.0% oftheory, which had a purity of 98% after fine distillation.

Analysis: ³¹P-NMR (CDCl₃): 2-methylpropyl phenylphosphinate 25.4 ppm

Example 5: 2-Methylpropyl phenylphosphinate (phenylphosphonous acidmono-2-methylpropyl ester)

Analogously to Example 3 above, diethyl phenylphosponite (purity 98%)was reacted with water (1 molar equivalent) and isobutanol (20 molarequivalents, which were added in two roughly equal portions) in thepresence of a catalytic amount of methanesulphonic acid (1% by weight,based on the amount of diethyl phenylphosphonite used).

2-Methylpropyl phenylphosphinate was obtained in a yield of 97.0% oftheory, which had a purity of 98.5% after fine distillation.

Analysis: ³¹P-NMR (CDCl₃): 2-methylpropyl phenylphosphinate 25.3 ppm

The invention claimed is:
 1. A method for producing

glufosinate, or a salt thereof, wherein a phosphonous acid monoester offormula (I)

is employed as an intermediate, the method comprising: (a) reacting acompound of formula (II)

with a compound of formula (III)R²—OH  (III) to produce a compound of formula (I); and (b) furtherreacting the compound of formula (I) with at least one further reactantto produce glufosinate or a salt thereof, wherein in each case: R¹represents (C₁-C₁₂)-alkyl, (C₁-C₁₂)-haloalkyl, (C₆-C₁₀-aryl,(C₆-C₁₀)-haloaryl, (C₇-C₁₀)-aralkyl, (C₇-C₁₀)-haloaralkyl,(C₄-C₁₀)-cycloalkyl or (C₄-C₁₀)-halocycloalkyl, R² represents(C₃-C₁₂)-alkyl, (C₃-C₁₂)-haloalkyl, (C₆-C₁₀)-aryl, (C₆-C₁₀)-haloaryl,(C₇-C₁₀)-aralkyl, (C₇-C₁₀)-haloaralkyl, (C₄-C₁₀)-cycloalkyl or(C₄-C₁₀)-halocycloalkyl, and R³ and R⁴ each independently of one anotherrepresent methyl or ethyl, in the presence of an acidic catalyst and inthe presence of water.
 2. The method according to claim 1, wherein thetotal amount of water used in process step (a) is at least 0.8 molarequivalents, based on the total amount of compounds of formula (II)used.
 3. The method according to claim 1, wherein the total amount ofwater used in process step (a) is 1 to 5 molar equivalents, based on thetotal amount of compounds of formula (II) used.
 4. The method accordingto claim 1, wherein the total amount of water used in process step (a)is 1 to 3 molar equivalents, based on the total amount of compounds offormula (II) used.
 5. The method according to claim 1, wherein the totalamount of water used in process step (a) is 1 to 2 molar equivalents,based on the total amount of compounds of formula (II) used.
 6. Themethod according to claim 1, wherein the total amount of compounds offormula (III) used in process step (a) is 1 to 25 molar equivalents,based on the total amount of compounds of formula (II) used.
 7. Themethod according to claim 1, wherein the total amount of compounds offormula (III) used in process step (a) is 3 to 15 molar equivalents,based on the total amount of compounds of formula (II) used.
 8. Themethod according to claim 1, wherein the pKa of the acidic catalystunder standard conditions is less than
 3. 9. The method according toclaim 1, wherein the acidic catalyst is selected from the groupconsisting of H₃PO₃, H₂SO₄, HCl, HBr, HClO₄, dichloroacetic acid,trichloroacetic acid, trifluoroacetic acid, methanesulphonic acid,trifluoromethanesulphonic acid, benzenesulphonic acid,p-toluenesulphonic acid, acidic ion exchangers, acidic polysiloxanes andacidic zeolites.
 10. The method according to claim 1, wherein thereaction in process step (a) is carried out as a one-pot reaction. 11.The method according to claim 1, wherein the reaction in process step(a) is carried out at a temperature in a range from 30 to 140° C. 12.The method according to claim 1, wherein the reaction in process step(a) is carried out in an inert diluent or diluent-free.
 13. The methodaccording to claim 1, wherein R¹ represents (C₁-C₆)-alkyl,(C₁-C₆)-haloalkyl, (C₆-C₈)-aryl, (C₆-C₈)-haloaryl, (C₇-C₁₀)-aralkyl,(C₇-C₁₀)-haloaralkyl, (C₅-C₈)-cycloalkyl or (C₅-C₈)-halocycloalkyl, andR² represents (C₃-C₈)-alkyl, (C₃-C₅)-haloalkyl, (C₆-C₈)-aryl,(C₆-C₈)-haloaryl, (C₇-C₁₀)-aralkyl, (C₇-C₁₀)-haloaralkyl,(C₅-C₈)-cycloalkyl or (C₅-C₈)-halocycloalkyl.
 14. The method accordingto claim 1, wherein R¹ represents (C₁-C₄)-alkyl, (C₁-C₄)-haloalkyl or(C₆-C₈)-aryl, R² represents (C₃-C₆)-alkyl or (C₃-C₆)-haloalkyl.
 15. Themethod according to claim 1, wherein the compound of formula (I) isfurther reacted in process step (b) to produce a salt selected from thegroup consisting of ammonium salts, phosphonium salts, sulfonium salts,alkali metal salts and alkaline earth metal salts of glufosinate. 16.The method according to claim 1, wherein the compound of formula (I) isfurther reacted in process step (b) to produce is glufosinate sodium orglufosinate ammonium.